Identifying the muscles used in different functional activities and evaluating the metabolic and hemodynamic responses to this activity are essential components of both basic and clinical studies of neuromuscular function. A recent approach to these questions is "muscle functional Magnetic Resonance Imaging" (mfMRI), in which active muscles appear with higher signal intensity (SI) in certain MR images than do inactive muscles. These SI changes result from an increase in the transverse relaxation time constant (T2) of muscle water and a Blood Oxygenation Level Dependent (BOLD) contrast. The physiological events that contribute to these phenomena combine to produce a complex SI time course that is characterized by 1) an initial rise, lasting through the first one or two images; 2) an early dip, extending from the end of the initial rise through the first approximately 60 s; and finally 3) a long-latency increase, which reaches a plateau after about 2 min. We propose that this time course is produced by a complex superposition of metabolic and hemodynamic events, which affect transverse relaxation through mechanisms such as chemical and diffusive exchange and variations in blood magnetic susceptibility. The overall goal of the proposed experiments is therefore to describe quantitatively the biophysical basis of the mfMRI SI time course, with the aim that such an understanding would clarify the relationships between MR imaging and physiological parameters of interest and thereby accelerate the development of mfMRI into practical applications. We will: 1) examine the effects of physiological variables on proton exchange between free intracellular water and intracellular proteins; 2) examine membrane permeability alterations during exercise in vivo; 3) quantify the BOLD effect's contribution to mtMRI SI changes; and 4) develop a comprehensive model that describes quantitatively how physiological and biochemical variables altered during exercise determine mfMRI SI. The outcome of these experiments will be a comprehensive and quantitative description of the physiological and biophysical influences on transverse relaxation in skeletal muscle in vivo, and how experimental variables (such as image type and timing) can be altered to enhance the sensitivity of mfMRI to particular physiological phenomena. Future applications of this knowledge may include studies of metabolic and hemodynamic responses of muscles to exercise in health and disease, the development of improved biomechanical models of functionally relevant multi-joint actions, and the evaluation of functional electrical stimulation protocols for patients with spinal cord injury.